You are trying to connect to your Mac or Virtual Machine using Parallels Mobile
via a 3G network
connection and get a black screen
This may happen in case if you have maximum Image Quality
set in the Parallels Mobile settings. 3G network sometimes cannot support maximum Image Quality due to low connection speed
in the comparison with Wi-Fi connection.
In order to solve this issue you need to reduce Image Quality
in Parallels Mobile settings. Please refer to screenshot below for more information: